Photoelectric device



PHOTOELECTIRIC DEVICE Filed Sept. 28, 1937 J :b AMPLIFIER I9 m. a

99292:- BY fl w ATTORNEY.

Patented Feb. 13, 1940 UNITED ,sTA-res rno'ronmc'rmc nnvion Paul S. Lester, East Orange, N. 1., assignor, by

mesne assignments, to

Radio Corporation of America, New York, N. Y., a corporation of Delaware Application September as, lat-1, Serial No. 168,053 3 Claims. (Cl. 250-153 My invention relates to television transmitting and electron image tubes, and more particularly to tubes utilizing separate photoelectrically sensitive cathodes and scanned targets.

5 In some television transmitting and electron the cathode are directed to a target or mosaic electrode having elemental areas with a high secondary emission coeflicient, or ratio of the number of secondary electrons emitted to the number of impinging electrons. For optimum 2 operation the target or mosaic electrode should have elemental areas with a secondary emission coemcient, which is high and is greater than unity. Such targets are, in general, photosensi Usually the target has a mosaic surface of tive. an individually separated and oxidized silver particles coated with caesium. Such a mosaic surface has the desired high secondary emission coefiicient, but is inherently photosensitive, so that when light falls on it photoelectrons are liberated.

In the operation of such a device the optical image to be transmitted is focused upon the semi-transparent photoelectric cathode, and some of the light is unavoidably transmitted through the translucent cathode and falls upon an the target or mosaic electrode. If the optical image is focused upon the semitransparent photoelectric cathode the transmitted light which falls upon the photoelectrically sensitive target or mosaic electrode forms an out-of-focus image 40 andelectrons are liberated from the target or mosaic electrode in accordance with this out-offocus image. Such emission of electrons impairs the desired fidelity of transmission.

One object of my invention is to provide a television transmitting tube which has a photoelectrically sensitive cathode facing a mosaic electrode and in which background effects due to light from the photocathode are substantially reduced or eliminated. Another object of my invention is to provide a cathode television transmitting tube of the type which has greater fidelity of transmission than has heretofore been obtainable in such tubes.

In accordance with my invention the semitransparent photocathode is so made that a negligible amount of the light falling on it is transmitted to the associated photoelectrically sensitive target or mosaic electrode. In one form of my invention the image of an object to be transmitted is focused on a translucent photoelectric cathode 'formed on a surface which is translucent and suflicently lightdifiusing to prevent an optical image being formed on an associated target or mosaic electrode. The light difl'using surface may be formed on an interior portion of the tube wall or on a light transparent member within the tube. The primary electron streams liberated from the surface of the cathode are directed to and focused upon the target or mosaic electrode which has individually separated particles having a high secondary emission cofllcient, the elements being periodically scanned by a cathode ray beam to discharge them and produce a useful output current.

A better understanding of my invention will be obtained and other objects, features and advantages of my invention will appear from the following description taken in connection with the accompanying drawing in which,

Figure l is a longitudinal view, in cross section, illustrating one form of television transmitting tube embodying my invention, and Figure 2 is a fragmentary view, in cross section and on a greatly enlarged scale, illustrating the ode shown in Figure 1.

Referring to Fig. l a tube made in accordance with my invention comprises a highly evacuated envelope or bulb I with an elongated cylindrical or body section and a neck section near one end of the envelope inclined at an acute angle with respect to the cylindrical section. The cylindrical section is closed at each end, the closure at the end opposite the neck section preferably being of optically uniform glass which is sealed to the body section of the envelope or bulb during the manufacturing process to form a window or end section 2 and which has thereon a semitransparent photoelectrically sensitive surface constituting the photocathode 3. oppositely disposed irom thephotocathode 3 is a target or mosaic electrode I positioned to receive photoelectrons which are liberated from the photocathode 3 and focused thereon by the electron focusing coil 5. An electron gun located in the neck of the bulb is provided to scan the mosaic electrode 4 with a cathode ray beam. The electron gun in the neck of the envelope comprises acathode 6, control electrode 1 connected to the I usual biasing battery, and a first anode 8 mainmosaic electrode 4by the second anode II which ,2 I tained. mt. with respect to the cathode by a battery 9 to produce a cathode ray beam controlled by the electrode projected through the anode U, and accelerated and concentrated into an electron scanning beam of the target or is preferablya conducting coating on-the inner surface of the envelope .1 in the neck oi' the bulb. I I

The mosaic electrode 4 is conventional, and

comprises a foundation l I of mica or other. insulating material havingon the side. facing the photoelectrically sensitive cathode 3 ;a multi-. plicity of mutually separated silver particles l2 which are oxidized and coated with caesiumto' provide a mosaic surfac'e'which exhibit'shigh secondary electron emissionwhen scanned by 'the cathode ray beam. The opposite side of "the foundation H is coated with a-layer I of-con ducting material such as platinum, which serves.

as a signal electrode and is connected to the grid of a translating device l4 and to ground through the impedance l5. Obviously the mosaic elecsided mosaic electrode is between the gun and the cathode 3.

Conventional beam deflection means such as the deflection coils I6 and H or conventional electrostatic deflection plates sweep the beam horizontally and vertically to scan the mosaic electrode 4. v

In accordance with my invention, as best shown in Fig. 2, I roughen the inner wall of the optically uniform and transparent end section 2 to provide a translucent light diffusing foundation surface l8 for the semi-transparent photoelectric cathode 3 which emits electrons when illuminated. I have found that such a surface preventsthe formation of an optical image upon the target or mosaic electrode 4, and that the photoelectric cathode should adjoin this surface. The inner wall of the glass end section 2 maybe roughened to provide the foundation surface l8 .by sand-blasting with finely-divided abrasive, by etching with acid, or by any other convenient method whereby a light diffusing surface may be obtained. If the transparent end section 2 is of lime glass it is preferably etched in such a manner as to obtain a roughened surface having a multitude of depressions or pits which are more or less rounded. This kind of etching may be done in two steps; first etching with a relatively strong ammonium bifluoride solution of relatively high acidity, and then re etching the surface thus formed with a weaker solution which rounds off the depressions and ridges formed by the first etch. A satisfactory etched surface is obtained by using for the first etch a solution containing the following constituents by weight:

Per cent Ammonium bifluoride 36 Dextrin (powder) 10 Barium sulphate 28 Sodium bisulphate Water 21 With this mixture, which contains about 12% hydrofluoric acid, the etching time is about 30 seconds at 50 C. For the second etching a water solution containing about 7/ hydrofluoric acid,

-2% de trin,=and-40% barium sulphate iskept in contact with the etched'glass, for about 30 seconds at 50C.- 'EIft he end section: is 'of'the hard low expansionglass commercially known as Pyrex glass,- I prefer to mechanlcally roughen the surface, for example, by sand-blast-- ins, preferably with finely-ground steel grit until the white light-transmission isreduced to about 90% of the original value before the blasting, and

then etch the {mechanically roughened surface with an acid solution of 5% hydrofluoric acid alsobeobtainedby immersing the surface in the etching muds.-' 1 1 Following the formation of theroughened sur-' face I thoroughly wash the surface with warm distilled water, The end vsectionis'then sealed into the. tube: and the tube. isevacuated. Silver is then evaporated within the tube and condenses on the surface 18 as a thin film of silver it which is sufficiently thin to be semi-transparent to light. ,The silver film is'then oxidized by subjecting the to a glow discharge in. the presence of oxygen until the film becomes practically transparent. Excessoxygen is pumped from the tube, caesium vapor is introduced into the tube, and the tube is baked for about five minutes at approximately 200 C. to sensitize the silver oxide film and form the photoelectric cathode 3. It has been found that with such treatment an excellent :electric cathode 3 to liberate streams of electrons which are directed to and focused upon the mosaic electrode 4 by the fousing coil 5 to form on the mosaic an electrostatic image of the optical image. The mosaic electrode is then scanned by the cathode ray beam generated by the electrongun in the neck section of the tube to neutralize the electrostatic image and produce signailing impulses representative of the electrostatic image.

It will be seen that I have provided a foundation for a photoelectrically sensitive cathode which diil'uses the light and prevents the formation of an out-of-focus image on the photosensitive target electrode 4, and that by the use of such a light diffusing foundation it is possible to obviate many of the disadvantages normally attendant upon the operation of television transmitting tubes incorporating a photoelectrically sensitive cathode and a separate mosaic electrode.

From the foregoing description it will be apparent that various other modifications may be made in my invention without departing from the spirit and scope thereof and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and set forth in the appended claims.

I claim:

1. A television transmitting tube including in an evacuated envelope having a transparent window with an etched light diffusing inner surface a photoelectrically sensitive mosaic electrode oppositely disposed from said window, and a photoelectric cathode adjoining said surface and between said mosaic electrode and said surface.

2. A television transmitting tube including a target electrode capable of emitting electrons when illuminated, a semi-transparent photoelectric cathode opposite and parallel to said target electrode, means to form an optical image on said cathode, and means consisting of an etched vitreous base between said first-mentioned means and said cathode and adjoining said cathode to diffuse the light of the optical image and prevent its falling on said target electrode as an out-oifocus replica of the optical image.

3. A television transmitting tube including an evacuated envelope with a, portion etched so as to be light diflusing, a light sensitive mosaic electrode adapted to emit secondary electrons when bombarded with high velocity electrons, a semitransparent photoelectric cathode adjoining said etched light diffusing portion oppositely disposed from and facing said mosaic electrode and between said etched light diflusing portion and said mosaic electrode.

PAUL S. LESTER. 

